1. Field of the Invention
The present invention relates to a recording apparatus for recording an image on a sheet and a method for controlling such a recording apparatus, and more particularly, it relates to a technique suitable for a recording apparatus of so-called serial scan type for scanning a recording head in a direction perpendicular to a sheet conveying direction.
2. Related Background Art
In conventional recording apparatuses such as printers, copying machines and facsimiles or conventional recording apparatuses used as composite electronic equipment including a computer or a word processor, or a work station, an image is recorded on a sheet such as a paper sheet or a plastic thin film.
Such recording apparatuses can be divided, according to recording types, into ink jet type, wire dot type, thermal type and laser beam type. In recording apparatuses of serial type using a serial scanning system in which the recording is effected by scanning a recording head in a direction (main scanning direction) perpendicular to a sheet conveying direction (sub scanning direction), an image is recorded by a recording head mounted on a carriage shifted along the sheet. After one-line recording is finished, the sheet is conveyed by a predetermined amount (pitch feed or sub scanning) by means of a convey means, and then, next one-line recording is effected with respect to the stopped sheet. By repeating such operations, the entire sheet is recorded.
On the other hand, in recording apparatuses of line type in which the recording is effected only by the sub scanning in the sheet conveying direction, the sheet is set at a predetermined recording position. And, after one-line recording is effected collectively, the sheet is conveyed by a predetermined amount (pitch feed), and then, next one-line recording is effected collectively. By repeating such operations, the entire sheet is recorded.
Among the above-mentioned recording types, a recording apparatus of ink jet type (in jet recording apparatus) in which the recording is effected by discharging ink from the recording head toward the sheet had advantages that the recording means can easily be made compact, a highly fine image can be recorded at a high speed, the recording can be effected to a normal sheet without special treatment, running cost is cheap, recording noise is small because of non-impact system, and a color image can easily be recorded by using multi color inks.
Particularly, in a recording apparatus of ink jet type in which the ink is discharged by utilizing thermal energy, since a recording head having high density liquid passage arrangement (discharge opening arrangement) can easily be manufactured by forming electrical/thermal converters formed on a substrate, electrodes liquid passage walls and a top wall through semiconductor manufacturing process such as etching, deposition and sputtering, the apparatus can be made more compact.
Now, an example of the conventional recording apparatus of serial scan type will be explained with reference to FIGS. 9 and 10. In FIG. 9, a pinch roller 102 is urged against a convey roller 101 for conveying a sheet P. A sensor lever 103 rockable around a shaft 103a is disposed at an upstream side (in a sheet conveying direction) (referred to merely as "upstream side" hereinafter) of the convey roller 101. When the sheet P abuts against a flag 103b of the sensor lever 103 to rotate the sensor lever 103 around the shaft 103a, a tail end detection sensor 104 is operated by a sensor operating portion 103c of the sensor lever 103, thereby detecting passage of the sheet P.
At a downstream side (in the sheet conveying direction) (referred to merely as "downstream side" hereinafter) of the convey roller 101, a recording head 105 for recording an image in response to image information is mounted on a carriage shiftable in a direction (main scanning direction) perpendicular to the sheet conveying direction, and a platen 106 for supporting a rear surface of the sheet P is opposed to the recording head 105.
A discharge roller 108 rotated at the same peripheral speed via a transmission roller 107 is disposed at the downstream side of the recording head 105, and a spurred roller 109 is urged against the discharge roller 108.
With the arrangement as mentioned above, in a condition that a tail end Pb of the sheet P is conveyed the downstream side of a nip n between the convey roller 101 and the pinch roller 102, since the sheet P is spaced apart from the convey roller 101, the sheet is subjected to only a conveying force and pinching and supporting action from the discharge roller 108 and the spurred roller 109.
Accordingly, in this condition, since the tail end Pb of the sheet P is not held down, an optimum gap between the recording head 105 and the sheet P is not established due to the floating of the sheet P. If the image is recorded by the recording head 105 in this condition, the recording of a high quality image cannot be expected. Further, if the tail end Pb of the sheet P is curved toward the recording head 105 or is curled, the sheet P is contacted with the recording head 105, thereby contaminating the sheet.
That is to say, generally, in the recording apparatus of serial scan type, after the tail end Pb of the sheet P has passed through the nip n between the convey roller 101 and the pinch roller 102, it is difficult to ensure the quality of the image.
Nevertheless, when the usage as a recording apparatus such as a facsimile or a copying machine is considered, there is strong request that the image is recorded on the entire recording surface of the sheet P. To this end, as shown in FIG. 10, there has been proposed a technique in which there are provided a floating preventing portion 106a (provided on the platen 106) for holding down an upper surface of the sheet at one lateral edge Pr (convey reference side) thereof along the sheet conveying direction to prevent the lateral edge Pr from floating, and a jam preventing rib portion 105a (provided on a lower surface of the carriage on which the recording head 105 is mounted) for preventing the sheet jam caused if a floating opposite lateral edge Pl (opposite to the lateral edge Pr) of the sheet abuts against the recording head 105.
With this arrangement, since the lateral edge Pr of the sheet P is pinched between an upper surface of the platen 106 and the floating preventing portion 106a, at least a distance {L.sub.1 -L.sub.2 } shown in FIG. 10 is maintained to separate the sheet from the nozzle surface of the recording head 105. Further, if the other lateral edge Pl of the sheet P is floating, since the floating lateral edge Pl is retarded from the recording head 105 by the jam preventing rib portion 105a provided on the carriage on which the recording head 105 is mounted, the sheet jam caused by abutting the recording head 105 against the sheet P can be prevented.
Recently, there has been strong request for effecting the recording with respect to various kinds of sheets, and the recording apparatuses has to cope with such request. However, in the above-mentioned conventional example, when a sheet P having very strong rigidity, if a tail end Pb of the sheet P is curved upwardly and the recording is effected up to the vicinity of the tail end Pb of the sheet P, during a return stroke of the recording head 105 after the scanning, there is a danger for abutting the other lateral edge Pl of the sheet P against the jam preventing rib portion 105a provided on the carriage to prevent the shifting movement of the carriage on which the recording head 105 is mounted.